Checking U-joint for wear - vibration at driveshaft

[dwkfym]

Hi,
Over the winter, I fixed a loose strut and replaced the shaft bearing.  Also installed a max-prop.  But like a dumb a** I reused the old coupler.  I also didn't check the shaft for any bends, except for a very casual look down the shaft where it looked straight. 

When I re-launched, the shaft had absolutely no movement at idle, so I thought I was good to go.  But at cruising RPMs and above, there is enough vibration to shake both the shaft and the V drive itself.  Its not much, but its visible.  I'd say about 1.5mm of play.  I don't think there is any discernable movement at the U joint, but with the V drive itself shaking, I can't tell which part of the drivetrain is causing the shake.

I re-aligned the V drive the best I could, and I can now get it up to 1850rpms without any visible movement.  I imagine a bent shaft or a bad coupler will show up at any RPMs.  So now I am wondering if I have a worn out U joint. 

How could I check the U-joint?  Any chance that lubing it will fix the problem?  Any way to check it without removing the U joint?  Thank you in advance. 

[SVJourney]

"But like a dumb a** I reused the old coupler."  Coupler?

A u-joint will be visible when it fails.  The little pins inside will disintegrate. so the joint will be loose, visible rust etc. Grab the shaft and look for movement. I don't think there is a zirc fitting on this one so you would have to remove the caps to lubricate.  Lubing it won't fix the problem, it just prolongs the life of the joint.

V-drive alignment is critical. You need to be within +/-.003 inch of perfect.  There are numerous how to threads on this site.

1850 rpms?  Is that at full throttle?

[dwkfym]

thanks for the response.  By coupler, I meant the part that bolts into the V drive and interfaces with the shaft via key and surface friction.  I aligned it using the 0.003 feeler gauge.  I had a buddy move around the V drive until I felt equal resistance all around the coupler and flange.  I will give it another shot - go with 0.002 feeler gauge, until I can feel equal resistance all around, and until I cannot insert a 0.003 feeler gauge at any spot. 

1850 RPM is when my vibration starts.  At full throttle the engine will rev to something like 2800rpms.  At 2100 rpms I am getting 6 knots at flat seas.  Prop is a max prop, balance checked at the factory, set at 17x10.4 pitch (the 3 blade I removed was 17x10). 

I do have a zerc fitting at my U joint, but I am not sure if it will lube the pins.  I haven't taken a good look at it because its underneath a piece of the cabin floor.  I suppose I should take it all apart and inspect, lubricate. 

[dwkfym]

I think I figured this one out.  Its my zinc.  I have 2 zincs on the shaft.  One is by the strut, other mid point of the shaft, to help protect the engine better.  This is probably what is causing the wobble at higher RPM.  It makes sense now, since there is no wobble or any perceptible movement at lower rpms.  Its whipping the shaft as the forces increase by the imbalance caused by the zinc. 

[dwkfym]

Update - the removal of the center zinc (one further away from the strut) did not solve the problem. 
I am going to try to loosen up my stuffing box hose on both ends, rotate them around a little bit before re-tightening them in case there is some sort of bind.  If that doesn't fix it, I probably have a bent shaft.